The present invention relates to a power conversion circuit, and more particularly to a power conversion circuit that is formed by a semiconductor device and includes a gate drive circuit for driving the gate of a transistor that functions as a switching element. The present invention also relates to a multiphase voltage regulator having the power conversion circuit, and to a power conversion method.
In a related art power conversion circuit, a power MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) or an IGBT (Insulated Gate Bipolar Transistor) is used as a switching element that provides power conversion control. Meanwhile, a power MOSFET based on gallium nitride (GaN) (hereinafter referred to as the GaN transistor) has attracted attention in recent years because it exhibits a high switching speed. The GaN transistor is divided into two types depending on the magnitude of a threshold voltage Vth: a normally-on type and a normally-off type. A normally-on transistor has a threshold voltage of lower than 0 and turns on when its gate voltage and source voltage are on the same potential. A normally-off transistor has a threshold voltage of higher than 0 and turns off when its gate voltage and source voltage are on the same potential.
The normally-on transistor is superior to the normally-off transistor in switching performance. For example, the normally-on transistor has a lower gate capacitance than the normally-off transistor and can reduce an on-resistance and perform a high-speed switching operation. However, when turning off the normally-on transistor, it is necessary that a negative voltage lower than a ground potential (0 V) be supplied to its gate. Therefore, when driving the normally-on transistor, it is necessary to prepare a negative voltage source, namely, a circuit that generates a negative voltage by using a charge pump or the like.
A semiconductor device that uses a normally-on transistor as a switching element is described in Japanese Unexamined Patent Publication No. 2011-101217. The semiconductor device described in Japanese Unexamined Patent Publication No. 2011-101217 includes a high-side drive for driving the gate of a normally-on GaN transistor in accordance with the negative voltage source.